Check out "Modifying & Recompiling Source Characters" on design3 and learn about Source engine character creation.
In this tutorial you'll learn the specific methods for extracting, decompiling, modifying and finally recompiling character and prop models in Half-Life 2. Using a powerful combination of the XSI Mod Tool, various third party apps and Valve’s own SDK.
Please help me! Im trying to make some new animations for the half life zombie model, I´ve been extracting the smd files from the mdl file, but when i pack the smd files back to a mdl file, the zombie model just stand still, it doesn´t walk at all, even when i just unpack the mdl file and then pack it back again without changin anything. It just stands there. Why? Please help me!!